You have multiple Microsoft Lync Server 2013 Front End servers in a pool.
You have multiple Lync Server 2013 Front End pools that have pool pairing enabled.
You move users back and forth between primary and backup pools by using the Move-CsUser PowerShell cmdlet or through the Lync control panel. (This includes a scenario where the user is moved to a pool that is not a backup pool and then moved back to the original pool.)
Either of the following issues occurs during the scheduled maintenance window:
Lync Server 2013 Front End servers in a pool is restarted.
Lync Server 2013 Front End pool is failed over and then failed back.
In this scenario, certain users may experience data loss on their client.
Note please call into Support for guidance if data loss is experienced even after you apply this update.
This issue occurs because the RG data (Routing Group) in the rtclocal database may be overwritten by an incorrect PSD (PersistedServiceData, which is a log of all the transactions performed by a user that are mapped to an RG). If the users are moved back to a pool they are previously homed on, their RG assignment may change, and the pool may recognize the pre-existing version of the user data instead the new one.